1. Field of the Invention
The present invention relates to papermaking machines and, more particularly, to papermaking machine configured to selectively recirculate exhaust air from a dryer so as to increase dewatering efficiency in processes upstream of the dryer, to reduce emissions from the papermaking machine, and to enhance a vacuum system associated with the papermaking machine.
2. Description of Related Art
Drying devices such as, for example, through-air dryers and Yankee dryers, are often employed in papermaking machines for drying a paper web after the paper web has been formed. Such drying devices often use a combination of heat and flowing air to dry the paper web and, as such, the exhaust from such drying devices comprises moisture-laden hot air. Generally, the venting of the exhaust from a drying device to atmosphere is undesirable for several reasons. For example, venting of the hot, moisture-laden air releases thermal energy that could be applied to other processes within the papermaking machine. Further, releasing the hot, moisture-laden air may increase undesirable papermaking plant emissions and may be unfavorably received by or may adversely affect neighbors surrounding the papermaking plant. In addition, significant and continuous environmental testing associated with the emissions may also be required. Accordingly, it would be desirable to reduce, minimize, or eliminate the emission of exhaust from such papermaking machine drying devices.
In some instances, the papermaking machine may be configured such that the exhaust from the drying device is recirculated through the drying device in order to reduce the heat input necessary to provide the heated air to the drying device, as well as to reduce emissions. In other instances, some of the exhaust from the drying device may be used to reduce process heat demands or to heat buildings. However, the heat from the exhaust of the drying devices often exceeds the amount of heat that can practically be re-used. In addition, a certain amount of the exhaust from the drying device must often be diverted so as to, for instance, remove excess condensates from the exhaust, wherein the exhaust may then be recirculated through the drying device. In such instances, though, the diverted portion may still be vented to atmosphere and thus will continue to undesirably contribute to plant emissions.
In order to reduce the amount of moisture to be removed from the web by the drying devices, many papermaking machines employ vacuum devices prior to the drying devices for partially dewatering the web. However, for example, in papermaking machines employing through-air dryers, it often undesirable to press or compact the web, though the web must still be dewatered to, for instance, about 18% to about 32% dryness. The vacuum devices thus employed to provide the necessary vacuum for dewatering the web to such an extent, and without pressing the web, often undesirably consume a significant amount of energy.
Thus, there exists a need for a papermaking machine having reduced emissions from the exhaust of the drying device(s). Further, it would be desirable for such a papermaking machine to have an efficient non-compacting (in the case of a machine employing a through-air dryer) dewatering process before the web is directed through the drying device(s). In addition, it would be desirable for the papermaking machine to exhibit reduced energy consumption with respect to the vacuum system and/or other high energy-consumption systems associated with the machine.